Misc Multi-Engine Considerations

This page consists of a few random multi-engine tidbits you'll need to understand for your checkride that didn't fit anywhere else.

Zero Fuel Weight

Zero fuel weight is the maximum allowable weight of the airplane before the addition of usable fuel. If the aircraft has a zero fuel weight, all weight added above it must consist of usable fuel only. The purpose of the zero fuel weight is to limit the load forces on the wing spars.

Minimum Equipment List

A Minimum Equipment List (or MEL) is a list compiled by an operator and approved by the FAA which lists equipment that may be inop and the flight still be permissible. A MEL is approved for a specific airplane and a Letter of Authorization is issued by the FAA. The MEL and the MEL constitute a Supplemental Type Certificate (STC).

A MEL cannot include items required to be operational by airworthiness requirements in the FARs under which airplane was certified, an airworthiness directive, or Part 91.

In the absence of a MEL, the following govern whether equipment must be operational for flight:
  1. Kinds of Operations list in the POH
  2. FAR 91.205
  3. Airworthiness directives
  4. Requirements of FAR under which airplane was originally certified

If the equipment is required by any of the above, you must have it fixed before further flight. Otherwise, you may placard the equipment inoperative and fly.

Why a Twin Is Not Twice as Fast

Assume a single has 300 lbs of drag at 150 knots. Double the thrust by adding a second engine. We know that thrust must equal drag, so the drag now is 600 lbs. Drag increases as the square of speed, so to see how much speed is added we take the square root of the additional thrust (sqrt(300) = 17) and we see the airplane is, theoretically, 17 knots faster.